Many vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Most engines are capable of operating efficiently only within a narrow range of speeds. Consequently, transmissions capable of efficiently transmitting power at a variety of speed ratios are frequently employed. When the vehicle is at low speed, the transmission is usually operated at a high speed ratio such that it multiplies the engine torque for improved acceleration. At high vehicle speed, operating the transmission at a low speed ratio permits an engine speed associated with quiet, fuel efficient cruising.
Automatic transmissions may include a torque converter for coupling a transmission input shaft to the crankshaft of the engine. To improve power transfer efficiency once the vehicle reaches a sufficient speed, a controller may engage a bypass clutch to selectively couple the transmission input shaft to the impeller of the torque converter, which is fixed to the flywheel of the engine. The bypass clutch may include one or more clutch plates that rotate with the housing and interleaved with one or more plates that rotate with the input shaft. To engage the clutch, pressurized fluid forces a piston to compress the clutch plates.